Hm. Some ideas for post Segarite Core unlocks:
First thing that jumps to mind; eliminating this:
restriction. So Nepgear keeps any excess Health between fights.
My next thought is something that improves Base Resilience. Segarite Core makes Health grow like crazy so it would make sense for some kind of ability that temporarily increase Base Resilience. I'm thinking a spell like Sega's Shield Charge but instead of adding-to/regenerating Health it adds to Base Resilience.
Ah but does being overpowered via an Intercept count as being negated. I chose to air on the side of caution and assume it doesn't for that calc.
I ask you; Why would we want such a thing?
Besides Celestial Severance hits planet killing size at Level 20 anyway. 26,214,400 Base Damage x2 from Double-Cast to 52,428,800 and on average another x2 from Echo to 104,857,600. Even without Echo it would probably be enough thanks to Unstoppable.